The present invention relates generally to the control of rolling mills and particularly to mill controls that compensate for changes occurring in rolling parameters that result from changes occurring in mill speed.
Changes in the rate of which a rolling mill reduces the thickness of a material directed through the mill, such as occurs when a mill is accelerated or decelerated, causes significant changes in the parameters of the rolling process, for reasons explained below. These parameters include the force at which mill rolls engage material in the roll bite of the mill, the friction in the roll bite, the torque at which the rolls direct the material through the roll bite, etc. Changes in such rolling parameters hamper the ability of the mill to produce consistent sheet thickness and flatness, which are quality concerns and thus the concern of the producer for his customers.
Generally, desired product quality is maintained by the use of reference values that are supplied to all mill actuators that control rolling parameters. These parameters include relative mill speed, average gap sizes, gap size differentials, average roll bending pressures, and differential roll bending pressures. The reference values, when properly set and adjusted, generally maintain desired product quality throughout small changes in mill speed.
Traditionally, closed loop, feedback control systems measure quality parameters, such as thickness and flatness downstream from the location of the roll gap where thickness and flatness disturbance are created. A required change in actuator setting is then calculated, and appropriate reference signals supplied to the respective actuators to correct thickness and flatness disturbances after the fact. Such adjustments are capable only of reducing, but not eliminating, parameter disturbances because of the delay in making corrections. The delay problem can be solved by using open loop, feedforward techniques, but these depend upon very accurate on-line mill models. Such models are expensive and require significant computational power. Further, mill conditions are not easily predicted and vary slowly over time. These aspects of rolling have not to date been accurately modeled yet they are associated with significant variations in critical rolling parameters as a result of mill speed changes. As a compromise, the rate of mill acceleration or deceleration is reduced on most mills today, as a slow pace in bringing the mill up to speed or slowing the mill down reduces the rate of parameter changes due to mill speed changes and, hence, allows the feedback controllers to more effectively reduce variations in critical rolling parameters.